Abstract INTRODUCTION Glioblastomas harbor inter and intratumoral genetic diversity, posing a challenge for targeted therapies. A major question is whether shared mechanisms might control the malignant phenotypes of genetically diverse glioblastoma cells. We reasoned that ubiquitin-dependent regulation of pluripotency-related transcription factor SOX2, which is indispensable for the maintenance of tumorigenic glioblastoma stem-like cells (GSC), may represent one such mechanism. TRIM26, an E3-ubiquitin ligase with immune-related functions, is highly expressed in glioblastoma tumors compared to normal brain. Immunoprecipitation followed by mass spectrometry suggested TRIM26 interacts with SOX2. We hypothesized that TRIM26 plays an essential role in GSCs by regulating SOX2 function. METHODS Direct protein-protein interactions were assessed by in vitro binding assays. In Vitro ubiquitination assays were performed. Lentiviral TRIM26 overexpression and knockdown were used to test the role of TRIM26 in regulating SOX2 stability, activity, and ubiquitination. The functional relevance of TRIM26 in GSCs was assessed by in Vitro self-renewal and in Vivo tumorigenicity assays. RESULTS We found that TRIM26 directly interacts with SOX2 via the C-terminal PRY-SPRY domain. Unexpectedly, TRIM26 overexpression resulted in decreased SOX2 polyubiquitination in cells. In line with this observation, TRIM26 knockdown in GSCs decreased SOX2 protein stability without changing SOX2 mRNA levels. Functionally, TRIM26 knockdown reduced SOX2 transcriptional activity, self-renewal, and in Vivo tumorigenicity in multiple genetically divergent GSC lines. Mechanistically, we discovered TRIM26 stabilizes SOX2 protein by competitively reducing the interaction of SOX2 with WWP2, a bonafide SOX2 E3 ligase in GSCs. Accordingly; WWP2 depletion in the setting of TRIM26 knockdown in GSCs rescued SOX2 protein levels, self-renewal, and tumorigenicity. CONCLUSION Together, these results suggest that TRIM26 maintains GSCs by protecting SOX2 from WWP2-mediated ubiquitination and subsequent proteasomal degradation. These findings raise the intriguing possibility that modulating ubiquitin-dependent regulation of SOX2 in genetically heterogeneous GSCs may represent a unifying therapeutic strategy.